Data rate adaptation in multiple-in-multiple-out systems

ABSTRACT

Systems and techniques relating to wireless communication are described. A described technique includes transmitting a first signal wirelessly to a wireless communication device in accordance with a first transmit mode that is selected from a plurality of transmit modes; receiving a shortlist from the wireless communication device, the shortlist identifying a subset of the transmit modes, the subset of the transmit modes including two or more modes that are different from the first transmit mode; selecting a second transmit mode from the shortlist; transmitting a second signal wirelessly to the wireless communication device in accordance with the second transmit mode; and selectively cycling through any remaining modes of the shortlist based on a lack of reception of an acknowledgement to the second signal. The wireless communication device can be configured to generate the shortlist based on a channel quality analysis of a received version of the first signal.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of, and claims priority to, U.S.patent application Ser. No. 13/425,350, filed on Mar. 20, 2012, entitled“Data Rate Adaptation In Multiple-In-Multiple-Out Systems” (now U.S.Pat. No. 8,532,081), which is a continuation of U.S. patent applicationSer. No. 10/620,024, filed on Jul. 14, 2003, entitled “Data RateAdaptation in Multiple-In-Multiple-Out Systems” (now U.S. Pat. No.8,149,810), which claims priority to U.S. Provisional Application No.60/447,448, filed Feb. 14, 2003, entitled “Data Rate Adaptation inMultiple In Multiple Out (MIMO) Systems”. The application herein claimsthe benefit of priority of all of the above listed patent applicationsand hereby incorporates by reference in their entirety the said patentapplications.

BACKGROUND

Wireless phones, laptops, PDAs, base stations and other systems maywirelessly transmit and receive data. A single-in-single-out (SISO)system may have two transceivers in which one predominantly transmitsand the other predominantly receives. The transceivers may use multipledata rates depending on channel quality.

An M_(R)×M_(T) multiple-in-multiple-out (MIMO) wireless system, such asthat shown in FIG. 1, uses M_(T) transmit antennas 104 at a firsttransceiver 100 and M_(R) receive antennas 106 at a second transceiver102. First and second transceivers 100, 102 in FIG. 1 are designated“transmitter” and “receiver”, respectively, for the purposes ofillustration, but both transceivers 100, 102 may transmit and receivedata.

The multiple antennas 104, 106 may improve link quality (e.g., achievinga minimum bit error rate (BER)) by using a transmission signaling schemecalled “transmit diversity,” where the same data stream (i.e., samesignal) is sent on multiple transmit antennas 104, after appropriatecoding. The receiver 102 receives multiple copies of the coded signaland processes the copies to obtain an estimate of the received data.

The multiple antennas 104, 106 may achieve high data rates by usinganother transmission signaling scheme called “spatial multiplexing,”where a data bit stream may be demultiplexed into parallel independentdata streams. The independent data streams are sent on differenttransmit antennas 104 to obtain an increase in data rate according tothe number of transmit antennas 104 used.

SUMMARY

The present application relates to a hybrid multiple-in-multiple-out(MIMO) system that may use aspects of link adaptation and cyclingthrough a shortlist of transmit modes. A receiver may receive signalsfrom a transmitter and derive channel quality statistics, such as a meansignal-to-interference-and-noise ratio (SINR). The receiver may comparethe derived post-processing mean SINR with pre-determined threshold meanSINRs in a lookup table for an ideal “orthogonal” channel. The receivermay use the lookup table to efficiently find an optimum transmissionmode or a shortlist of possible modes. The single lookup table may berelatively small and more efficient to implement than one or more lookuptables that account for multiple channel scenarios. The receiver feedsback the optimum transmission mode or shortlist to the transmitter,which adapts its spatial multiplexing rate s, coding rate r andmodulation order m. The receiver may feed the optimum transmissionscheme(s) back to the transmitter in a compressed manner, such as lookuptable indices. The system may minimize retransmissions and hence improvemedium access controller (MAC) throughput.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a wireless multiple-in-multiple-out (MIMO)communication system, which includes transceivers with multipleantennas.

FIG. 2 is a flowchart of a method of using the system of FIG. 1.

FIG. 3 illustrates an embodiment of the receive portion and transmitportion of the receiver in FIG. 1.

FIG. 4 illustrates operations of the transmitter in FIG. 1.

FIG. 5 illustrates another embodiment of the receive portion andtransmit portion of the receiver in FIG. 1.

FIG. 6A illustrates an example of the lookup table in FIG. 5.

FIG. 6B illustrates another example of the lookup table in FIG. 5.

FIG. 7 illustrates a graph of measured signal-to-interference-and-noiseratio (SINR) as it fluctuates over time.

FIG. 8 illustrates a table of time diversity orders and correspondingpercentages of time that measured SINR fades 10 dB below the mean SINRin FIG. 7.

FIG. 9 is a rate vs. distance graph of a 1×1 single-in-single-out (SISO)system and a 3×3 MIMO system.

DETAILED DESCRIPTION

FIG. 1 illustrates a wireless multiple-in-multiple-out (MIMO)communication system 130, which includes a first transceiver 100 withmultiple antennas 104 and a second transceiver 102 with multipleantennas 106. In an embodiment, each transceiver has four antennas,forming a 4×4 MIMO system. For the description below, the firsttransceiver 100 is designated as a “transmitter” because the transceiver100 predominantly transmits signals to the transceiver 102, whichpredominantly receives signals and is designated as a “receiver”.Despite the designations, both “transmitter” 100 and “receiver” 102 maytransmit and receive data, as shown by the transmit portions 101A, 101Band receive portions 103A, 103B in each transceiver.

The transmitter 100 and receiver 102 may be part of a MIMO-OFDM(orthogonal frequency division multiplexing) system. OFDM splits a datastream into multiple radiofrequency channels, which are each sent over asubcarrier frequency. The transmitter 100 and receiver 102 arepreferably implemented in LANs or WANs. It is also contemplated thatsuch transceivers may be implemented in any type of wirelesscommunication device or system, such as a mobile phone, laptop, personaldigital assistant (PDA), a base station, a residence, an office, etc.

The number of independent data streams transmitted by the transmitantennas 104 may be called a “multiplexing order” or “spatialmultiplexing rate” (s). A spatial multiplexing rate s=1 indicates purediversity, and a spatial multiplexing rate s=min(M_(R),M_(T)) (minimumnumber of receive or transmit antennas) indicates pure multiplexing. TheMIMO system 130 may use combinations of diversity and spatialmultiplexing, i.e., 1≦s≦min(M_(R),M_(T)), depending on a channelscenario.

Each data stream may have an independent coding rate (r) and amodulation order (m). The physical (PHY) layer, or raw, data rate may beexpressed as R=r×log₂(m)×s Bps/Hz. A transmitter's PHY layer chip maysupport many data rates depending on the values of s, r and m. Forexample, a 4×4 MIMO system with IEEE 802.11a coding and modulationschemes (8 in number) and 6 spatial multiplexing orders

$\left( {s \in \left\lbrack {\frac{1}{2},\frac{3}{4},1,2,3,4} \right\rbrack} \right)$may have up to 8×6=48 different data “transmission modes” or“transmission schemes,” each with its own data rate.

A “transmission mode” or “transmit mode” refers to a set of transmissionparameters, such as transmission signaling scheme (spatial multiplexing,transmit diversity or some combination), data rate R, coding rate r,modulation order m and modulation level, e.g., 8PSK (phase shiftkeying), GMSK (Gaussian minimum shift keying), BPSK (binary PSK), QPSK(quaternary PSK), 16-QAM (16-quadrature amplitude modulation, 64-QAM,etc.

The optimum data rate and transmission mode that achieve a target biterror rate (BER) may vary, depending on user locations and channelcharacteristics, which may cause time-selective fading,frequency-selective fading and space-selective fading.

The current IEEE 802.11 modem of a single-in-single-out (SISO)transmitter starts transmission at the highest possible data rate. Ifthe transmitter modem receives an acknowledgement (ACK) signal from aSISO receiver, then the modem uses the highest possible data rate forfurther transmissions. Otherwise, the modem lowers the data rate andcycles through all possible data rates until the transmitter receives anACK signal from the receiver. For a 4×4 MIMO system with 48 possibledata rates, the cycle-through method may waste a significant amount ofreal-time bandwidth.

“Link adaptation” (also referred to as “adaptive modulation”) may be amore efficient mechanism to select an optimum data transmission rate.Link adaptation has been studied for SISO systems. A receiver (a)measures a channel quality characteristic (also called channel qualitycondition or channel state information (CSI)) (e.g., signal-to-noiseratio (SNR) or signal-to-interference-and-noise ratio (SINR)), (b)converts the channel characteristic into BER information for eachtransmit mode, (c) selects an optimum transmit mode (from a plurality ofmodes) based on the channel characteristic and a target BER, and (d)feeds the channel characteristic or selected mode back to thetransmitter. The transmitter receives the fed back channelcharacteristics or selected mode and adjusts its transmit mode and datarate accordingly.

Link adaptation exploits variations of a wireless channel over time,frequency and/or space due to changing environmental and interferenceconditions by dynamically adjusting transmission parameters. Linkadaptation may select the most efficient mode for spectral efficiencyover varying channel conditions based on a mode selection criterion,e.g., maximum data rate, for each link. Since each transmit mode has aunique data rate and minimum SNR needed to activate the mode, differenttransmit modes are suited for optimal use in different channel qualityregions.

For example, the receiver may estimate a SINR for each channelrealization and choose a transmit mode (data rate, transmission codingrate r, and modulation order m). In an ideal link adaptation system, thereceiver receives a packet and instantaneously sends feedback indicatingthe chosen transmit mode to the transmitter. Fast feedback is not alwayspossible or desired because feedback consumes bandwidth. For systemswith a limited feedback rate (e.g., slower than the “channel coherencetime”, which is the time duration when channel impulse responses remainstrongly correlated), the receiver may receive a plurality of packetsduring a time window and obtain SINR “statistics” (e.g., mean SINR,variance, etc.) from the packets. The transmitter may use the SINR“statistics” to adapt the transmit mode.

A SISO receiver may create a plurality of lookup tables for a pluralityof channel scenarios. Each channel scenario may be characterized by SINRvariance across time (σ_(t) ²) and variance across frequency (σ_(f) ²).The receiver may index the lookup tables according to σ_(t) ² and σ_(f)². Each lookup table has a plurality of entries for a plurality ofdifferent transmit modes and their mean threshold SINRs ( μ _(r,m)). Themean threshold SINR is the minimum SINR required for a given transmitmode to operate at a target BER.

For each time window, which can span several received packets, thereceiver computes the mean SINR (μ) and SINR variances σ_(t) ², σ_(f) ².The receiver uses the SINR variances to select an appropriate lookuptable. Within the selected lookup table, the receiver uses the mean SINR(μ) to select a transmit mode (data rate, transmission coding rate r,and modulation order m) that maximizes the data rate while operating atthe target BER or packet error rate (PER). In other words,

$r^{*},{m^{*} = {\underset{r,m}{argmax}\left\lbrack {{{sgn}\left( {\mu - \overset{\_}{\mu_{r,m}}} \right)} \times s \times \log_{2}m \times r} \right\rbrack}}$where s−×log₂m×r is the data rate as described above. The BER may beextracted from the cyclic redundancy check (CRC) information at the linklayer. If multiple transmit modes yield the same data rate, then thereceiver or transmitter may select the transmit mode with the highestSINR margin

$\left( {\mu - \overset{\_}{\mu_{r,m}}} \right).$

A similar link adaptation scheme may be implemented in a MIMO system.However, such an approach may be difficult to implement since MIMOchannel scenarios and transmit modes are much larger than in a SISOsystem. The channel scenarios may be larger due to dependencies onproperties like LOS (line-of-sight) matrix, polarization, correlation,etc. For example, assuming 10 different channel scenarios and 48transmit modes for a 4×4 IEEE 802.11 MIMO system, the MIMO system mayrequire in a large lookup table with 48×10=480 entries (or 10 lookuptables each with 48 entries). If any transmission scheme is changed oradded, all 480 entries may have to be recomputed. Computing the lookuptable entries via numerical simulation or measurements for each channelscenario may be difficult and time-consuming.

Link Adaptation and Cycling Through a Shortlist

In an embodiment, the MIMO system 130 in FIG. 1 may use a hybrid schemethat combines aspects of link adaptation and cycling through a subset ofavailable transmit modes, e.g., a shortlist of best transmit modesdetermined at the receiver 102. The hybrid scheme may address the MIMOlink adaptation problems described above.

FIG. 2 is a flowchart of an exemplary method of link adaptation andcycling through a shortlist, which may be used by the system 130. Thetransmit portion 101A (FIG. 1) of the transmitter 100 sendssignals/packets 150 which are received by the receive portion 103B ofthe receiver 102 at 202 (FIG. 2). FIG. 3 illustrates an embodiment ofthe receive portion 103B and transmit portion 101B of the receiver 102.A channel quality circuit 130 in the receive portion 103B of thereceiver 102 may determine one or more channel quality statistics fromthe received signals 150 at 204. A subset selection module 114 in thereceive portion 103B compares the channel quality statistics to entriesin a lookup table 108 at 206. For example, each entry in the lookuptable 108 may store a transmit mode and its corresponding channelquality statistic, e.g., a threshold post-processing mean SINR

$\overset{\_}{\mu_{s,r,m}}$(described below). The subset selection module 114 selects a shortlistor subset of transmit modes from the lookup table 108 with channelquality statistics that are close to the measured channel qualitystatistics at 208. The transmit portion 101B of the receiver 102 sendsthe shortlist to the receive portion 103A of the transmitter 100 at 210.The shortlist feedback may be a part of an acknowledge/no acknowledge(ACK/NACK) packet.

FIG. 4 illustrates operations of the transmitter 100 in FIG. 1. Thereceive portion 103A of the transmitter 100 receives the shortlist ofmodes at 402. The transmitter 100 selects a transmit mode in theshortlist at 404 and finds a corresponding data rate in a lookup tablestored in the transmitter 100 at 406. The transmit portion 101A of thetransmitter 100 initiates further transmission at the found data rate(e.g., highest data rate selected from the shortlist of modes) at 408.If the transmitter 100 receives an ACK from the receiver 102 within apre-determined time period at 410, then further transmissions from thetransmitter 100 may occur at this data rate at 412. Otherwise, thetransmitter 100 lowers the data rate and cycles through the shortlist ofmodes at 414 until an ACK is received from the receiver 102 at 410.

If the transmitter 100 has tried every mode and its corresponding daterate in the shortlist 108, an alternative strategy may be used. Forexample, the transmitter may use a low data rate until the receiver 102sends another shortlist at 416. In alternative embodiments, thetransmitter may try other data rates or request a new shortlist from thereceiver 100.

In an embodiment, the receiver 102 may store pre-configured operatingmodes in a memory such as a non-volatile memory 109, which may includethe lookup table 108, as shown in FIG. 5. The lookup table 108 may haveentries for a plurality of transmission modes (e.g., 48 modes) andcorresponding mean threshold receive “post-processing” SINRs

$\left( \overset{\_}{\mu_{s,r,m}} \right).$A “post-processing” SINR refers to SINR derived after data from multipleantennas are combined, as opposed to a “pre-processing” SINR derivedfrom data at each antenna. A mean threshold receive post-processing SINR

$\left( \overset{\_}{\mu_{s,r,m}} \right)$is the minimum post-processing mean SINR required for a given transmitmode (i.e., set of spatial multiplexing rate s, coding rate r, andmodulation order m values) to operate at a target BER.

FIG. 6A illustrates an example of the lookup table 108 in FIG. 5. FIG.6B illustrates another example of the lookup table 108 in FIG. 5, inwhich the available modes are separated by different spatialmultiplexing rates, e.g., s=1, 2, etc. (described below).

In contrast to the proposed MIMO link adaptation scheme with multiplechannel scenarios described above (which used 10×48=480 entries), thereceiver 102 may store a relatively small lookup table 108 (e.g., 48entries) by assuming a selected ideal or generalized reference channel,such as an “orthogonal channel” or an Independent and IdenticallyDistributed (IID) channel. An IID channel may use a random MIMO matrix,where each element of the MIMO matrix is a complex, normal anddistributed mean of zero and a variance of one. An IID channel accountsfor fading margins. An “orthogonal” channel does not account for fadingmargins and may be equivalent to an additive white Gaussian noise (AWGN)channel for a SISO system. An orthogonal channel may be relatively easyto simulate. The receiver 102 may simulate both an IID channel and anorthogonal channel to see which one provides better results. The lookuptable 108 based on a selected ideal or generalized reference channel maybe relatively easy to generate and save. Thus, the receiver 102 does nothave to save lookup tables for a plurality of channel scenarios.

For example, in a 4×4 MIMO system, the lookup table 108 based on anorthogonal channel may have 48 entries for 48 modes and their thresholdpost-processing mean SINRs

$\left( \overset{\_}{\mu_{s,r,m}} \right),$as opposed to 480 entries for the MIMO link adaptation scheme describedabove.

The receiver 102 may receive a plurality of packets 150 from thetransmitter 100 during a time window, which may span several packets.The packets 150 may undergo processing, such as demodulation, and reacha channel estimator 120 and a noise and interference estimator 128. Foreach time window, the channel estimator 120 estimates a channelresponse, and the noise and interference estimator 128 estimates noiseand interference. Various techniques may be used for channel estimationand noise and interference estimation. For example, in an embodiment,each transmit antenna transmits a known training sequence that isorthogonal to the training sequences transmitted by the othertransmitters. The channel estimator 120 uses the received trainingsequence and known transmit sequence to estimate the channel element foreach transmit-receive antenna pair. For noise and interferenceestimation, the transmitter transmits null-sequences (e.g., zero tonesor empty packets). During this period, the receiver records signals foreach receive antenna. These signals are attributed to receiver noise andinterference. The noise variance (σ²) is the variance of the noise (andinterference) signal.

A channel quality indicator derivation module 111 derives a channelquality indicator, such as post-processing SINR, based on outputs of thechannel estimator 120 and noise and interference estimator 128. Thepost-processing SINR may be referred to as instantaneous or derived at aparticular instant in time.

For each time window, a channel quality statistics computation module113 computes channel quality statistics, such as a post-processing meanSINR μ_(s) and a “margin SINR”. The margin SINR is an increase in meanSINR desired to guard against channel fades. The margin SINR may includea “fading margin” and a “frequency diversity gain,” as described below.If the channel has no fades, then margin SINR=0. If the channel hassevere fading, then margin SINR may be a relatively large number. Thestatistics computation module 113 at the receiver 102 may generate themean receive post-processing SINR μ_(s) and margin SINR for each spatialmultiplexing rate s using the MIMO channel estimate from the channelestimator 122 and noise and interference power estimate from the noiseand interference power estimator 128 at the receiver 102.

The statistics computation module 113 may then compute an “outage SINR,”which is mean SINR+margin SINR. The subset selection module 114 uses thepost-processing mean SINR or preferably the outage SINR to find entriesin the lookup table 108 that have a threshold post processing mean SINRclose to the currently estimated mean SINR or outage SINR. The subsetselection module 114 may select a particular number of entries (e.g.,one to five), and send a shortlist of modes from the selected entriesback to the transmitter 100 via the transmit portion 101B.

The transmitter 100 may try sending data using the mode from theshortlist with the highest data rate at 406 in FIG. 4, and check for anacknowledgement (ACK) signal from the receiver 102 at 410. If thetransmitter 100 does not receive an ACK signal or receives ano-acknowledgement (NACK) signal, the transmitter 100 tries the nexthighest data rate of another mode in the shortlist at 414, 404. Thetransmitter 100 may eventually cycle through all data rates in all modesof the shortlist at 416.

There are at least two advantages of this system. The shortlist sent asfeedback from the receiver 102 to the transmitter 100 saves channelbandwidth because only a limited number of modes are sent instead of 48modes or other channel information that may require a number of feedbackmessages. In addition, the transmitter 100 saves time and resources bycycling through a limited number transmit modes and does not have tocycle through 48 transmit modes.

In an embodiment, the subset selection module 114 generates a shortlistwith three transmit modes. If the transmitter 100 tries all threetransmit modes and does not receive an ACK signal (e.g., because thereceiver 102 did not estimate a mean SINR or margin SINR accurately orthe channel conditions suddenly change), the receiver 102 may repeat theprocess of estimating a mean SINR and Margin SINR, finding three closestentries in the lookup table 108, and sending another shortlist to thetransmitter 100 at 210 in FIG. 2. The transmitter 100 may switch to alow data rate at 416 in FIG. 4 until the receiver 102 sends anothershortlist. Alternatively, the transmitter may try other data rates orrequest a new shortlist from the receiver 100, as described above.

The receiver 102 may be modified to provide instantaneous feedback tothe transmitter 100 without estimating the margin and mean SINR. Thetradeoff is between feedback rate and performance. Feedback based onSINR statistics (mean SINR and/or Margin SINR) may transmit at a lowerdata rate to guard against fluctuations in channel quality (as long asthe fluctuations were present in the time window of packets used toestimate SINR statistics). Instantaneous feedback of an optimum datarate may use up more “feedback” bandwidth but may transmit at theoptimum rate depending on the exact channel state.

The lookup table 108 may have a plurality of entries with thresholdpost-processing mean SINRs

$\left( \overset{\_}{\mu_{s,r,m}} \right),$which are the minimum SINRs required for available transmit modes. Eachtransmit mode may include a spatial multiplexing rate (e.g., 1, 2, 3,4), a coding rate r (e.g., the same as used by IEEE 802.11), and amodulation order m (e.g., 4QAM, 16 QAM, 64QAM, etc.). The descriptionbelow describes how the receiver 102 derives a post-processing mean SINRmeasurement, a fading margin or Margin SINR, a frequency diversity gainα_(r), and from these quantities, the outage SINR and “total OutageSINR.” The subset selection module 114 may then compare the outage SINRor total outage SINR with the lookup table entries to determine theshortlist of modes with their transmission rates.

Time-Selective Fading

FIG. 7 illustrates a graph of measured instantaneous SINR 702 as itfluctuates over time. The instantaneous post-processing SINRs will fadein time across packets. The mean SINR μ_(s) 704 indicates the averageSINR across time. A fading margin f_(s) indicates how much the SINR willfade below the mean SINR μ_(s), for a given x % of a time window. Ifthere is no fading, then f_(s)=0. If the channel is fast-fading, thenf_(s) may be a relatively large number, e.g., 10 dB. There may be anumber of ways for the statistics computation module 113 to compute afading margin f_(s) to account for fading. A specific method ofmeasuring this fading margin f_(s) is described below. The statisticscomputation module 113 or receiver software may compute the fadingmargin.

The statistics computation module 113 computes a probability expressedas P(SINR<μ_(s)−γ)=x %+/−ε% for each instantaneously measuredpost-processing SINR, a derived mean SINR μ_(s) (see FIG. 7) formultiple packets received in a time window, a target x %, e.g., 10%, adesired ε% indicating tolerance for measurement error, e.g., 1% or 2%,and different values of a variable γ=[0 . . . fs . . . MAX_VALUE] dB tofind an appropriate fading margin f_(s). For example, the statisticscomputation module 113 may try 3 dB, 6 dB, 9 dB, and 12 dB for γ in theequation above. As another example, the module 113 may try 2 dB, 4 dB, 6dB, and 8 dB for γ in the equation above. The statistics computationmodule 113 may use a histograph. Whichever value of γ that provides aprobability P closest x %+/−ε% is selected as the fading margin fs. Ifthe probability is ≈x % for a certain value of γ, then that γ isselected as the fading margin f_(s). For example, in one application,MAX_VALUE can be 20 dB, x=10%, ε=2%.

There may be different fading margins f_(s) for different multiplexingrates s. Each multiplexing rate s can exhibit an independent diversityorder d_(s), which may represent how much the post processing SINR willfade in time due to channel fades. For a 4×4 MIMO system 130 with fourspatial multiplexing rates s=1, 2, 3, 4, the statistics computationmodule 113 may determine four corresponding fading margins f_(s), onefading margin f_(s) for each multiplexing rate s.

FIG. 8 illustrates a table of time-diversity orders d_(s) andcorresponding percentages of time (x %) that measured SINR fades γ=10 dB(a selected and fixed value) below the mean SINR μ_(s) 704 in FIG. 7.The second column in FIG. 8 represents the “x” % variable describedabove. For example, a “first-order” time-diversity describes a postprocessing SINR that fades in time by γ=10 dB below the mean SINR μ_(s)x=10% of the time.

The “Outage SINR” may be expressed as:G _(s)=μ_(s) +fs

G_(s) represents a desired goal of estimated outage SINR and may be usedby the subset selection module 114 to find one or more entries in thelookup table 108. G_(s) is equal to the receive post-processing meanSINR μ_(s) plus the fading margin f_(s). The fading margin f_(s)represents how much the mean SINR μ_(s) will probably fade or fluctuateover time, and G_(s) is intended to account for this fading. If thefading margin f_(s) is large, then G_(s) will be large.

Another method of obtaining the fading margin f_(s) uses a knownexpression

$\left( {f_{s} = {\frac{10}{d_{s}} \times {\log_{10}\left( {x/100} \right)}}} \right)$relating outage probability and diversity order d_(s) at high SINRs. Thevalue “x” is a reliability factor (the second column in FIG. 8) obtainedaccording to an expression

${P_{b} \leq {0.5 \times \frac{x}{100}}},$where P_(b) is the target BER of the receiver 102. This equation assumesthat for (100-x) % of the time, G_(s) (x % outage SINR) will be greaterthan a threshold post-processing mean SINR

$\overset{\_}{\mu_{s,r,m}}$and have zero BER. While x % of the time, the received meanpost-processing SINR μ_(s) 110 will be smaller than the thresholdpost-processing mean SINR

$\overset{\_}{\mu_{s,r,m}}$and have 50% BER. For P_(b)=5×10⁻³, the above expression yields x=1%,providing 99% reliability.

The diversity order d_(s) may be computed by a number of methods. Onemethod for deriving diversity order d_(s) assumes a Rayleigh fadingchannel. “Rayleigh fading” refers to a transmitted signal that spreadsout, scatters and takes multiple paths of different lengths to arrive ata receiver antenna at different times. The ensemble of signals reflectedby the ground, bodies of water, atmosphere, etc., arrive at the receiverantenna and create standing waves. Multipath fading occurs. The methodcan approximate the receiver post-processing instantaneous SINR to be achi-square random variable with 2d_(s) degrees of freedom. It is knownthat for Rayleigh fading channels, an M_(R)×M_(T) system with spatialmultiplexing rate s≧1 has a diversity order ofd_(s)=(M_(R)−s+1)×(M_(T)−s+1). Theoretically, the receivepost-processing instantaneous SINR is then a chi-square random variablewith d_(s)=2×(M_(R)−s+1)×(M_(T)−s+1) degrees of freedom. However, inpractice, channel properties such as correlation, etc, can lead to asmaller diversity order, i.e., d_(s)<2×(M_(R)−s+1)×(M_(T)−s+1). Assumingthat the instantaneous SINR can still be approximately modeled as achi-square random variable with 2d_(s) degrees of freedom. Thestatistics computation module 113 may compute

${d_{s} = \sqrt{\frac{\mu_{s}}{\sigma_{s}}}},$where of is a variance of receive post-processing instantaneous SINR intime.

Frequency-Selective Fading

The receive post-processing instantaneous SINR may experiencefrequency-selective fading caused by a multipath channel. In such acase, the outer forward error correction (FEC) codes in packetstransmitted from the transmitter 100 may extract a “frequency diversitygain α_(r)” depending on the coding rate r. Frequency diversity gainα_(r) may be understood as follows. In a multipath orfrequency-selective fading channel, some tones of the OFDM system can bein a fade, while others may not. The FEC can recover or correct for thebits transmitted on the tones in a fade and improve the systemperformance. This improvement is called frequency diversity gain α_(r).This gain is measured by a decrease in Outage SINR required to obtain atarget system BER. Typically, the lower the target BER, the higher thefrequency diversity gain α_(r). The frequency diversity gains α_(r)provided by different FECs on a multipath channel at a certain BER, arewell-understood. With more frequency-selectivity, there will be morefrequency diversity gain α_(r). For a flat-fading channel, the frequencydiversity gain α_(r) is 0.

The frequency diversity gain α_(r) can be computed by the statisticscomputation module 113 via numerical simulations and stored in a lookuptable separate from the lookup table 108 in FIG. 5. For example, afrequency diversity gain lookup table may comprise coding rates of ½, ⅔and ¾ and corresponding frequency diversity gains of 3 dB, 2 dB and 1dB, respectively.

Using the Lookup Table

As stated above, the outage SINR may be expressed as G_(s)=μ_(s)+f_(s),where μ_(s) is the derived post-processing mean SINR and f_(s) is thederived fading margin. The statistics computation module 113 may combine(a) the outage SINR G_(s) (for each spatial multiplexing rate s, e.g.,s=1, 2, etc.) over time, with (b) the frequency diversity gain α_(r)(for each coding rate r) to obtain a “total Outage SINR” expressed asG_(s,r)=G_(s)+α_(r). The table look-up module 114 may use the totalOutage SINR G_(s,r) to find one or more entries in the lookup table 108with comparable post-processing mean threshold SINRs

$\overset{\_}{\mu_{s,r,m}}$to obtain one or more acceptable or optimum transmission modes. This maybe expressed as finding an acceptable or optimum coding rate r andmodulation order m for a given spatial multiplexing rate s:

$s^{*},r^{*},{m^{*} = {\underset{s,r,m}{argmax}\left\lbrack {{{{sgn}\left( {G_{s,r} - \overset{\_}{\mu_{s,r,m}}} \right)} \times s \times \log_{2}}{m \times r}} \right\rbrack}},$where “max” means maximum value, “sgn” means signum, and R=r×log₂(m)×sis the physical layer (raw) data rate (R_(s,r,m)) in Bps/Hz. If

${{{sgn}\left( {G_{s,r} - \overset{\_}{\mu_{s,r,m}}} \right)} = {+ 1}},{i.e.},{G_{s,r} > \overset{\_}{\mu_{s,r,m}}},$then s, r, m represent an acceptable mode for a shortlist. If

${{{sgn}\left( {G_{s,r} - \overset{\_}{\mu_{s,r,m}}} \right)} = {- 1}},{i.e.},{G_{s,r} < \overset{\_}{\mu_{s,r,m}}},$then s, r, m represent an unacceptable mode for a shortlist.

If multiple transmission modes yield the same data rate, then the subsetselection module 114 may choose the mode leading to the highest margin

$G_{s,r} - {\overset{\_}{\mu_{s,r,m}}.}$

Values Depend on Spatial Multiplexing Rate

In the channel quality statistics computation module 113 in FIG. 5,different spatial multiplexing rates s=1, 2, etc. may be associated withdifferent receiver types. If one stream of data is received (s=1), onetype of receiver may be used. If two streams of data are received (s=2),two different types of receivers may be used. For a 4×4 MIMO system,there may be 4 or 6 spatial multiplexing rates (e.g., s=½, ¾, 1, 2, 3,4). The statistics computation module 113 may be configured to determinea unique set of post-processing mean SINR μ_(s), margin SINR (includingfading margin f_(s) and frequency diversity gain α_(r)), and a totaloutage SINR G_(s,r) depending on which spatial multiplexing rate s isused by the transmitter 100.

For example, if the transmitter 100 uses spatial multiplexing rate s=1,then the statistics computation module 113 may determine a firstpost-processing mean SINR μ_(s), a first margin SINR (including a firstfading margin f_(s) and a first frequency diversity gain α_(r)), and afirst total outage SINR G_(s,r). If the transmitter 100 uses anotherspatial multiplexing rate s=2, then the statistics computation module113 may determine a second post-processing mean SINR μ_(s) a secondmargin SINR (including a second fading margin f_(s) and a secondfrequency diversity gain α_(r)), and a second total outage SINR G_(s,r).The subset selection module 114 then uses the first or second (s=1 or 2)total outage SINR G_(s,r) to access the lookup table 108, which may alsohave threshold mean SINRs and transmit modes categorized by s=1, s=2,etc., as shown in FIG. 6B.

The subset selection module 114 may use the computed outage SINR ortotal outage SINR to select a few transmission modes from the look-uptable 108, in decreasing order of data rate, to be in a shortlist andsent back to the transmitter 100.

The transmit portion 101B of the receiver 102 may send the transmitter100 a signal indicating the transmit modes in the shortlist, which mayadapt the data transmission rate, coding rate r and modulation order mof new data packets accordingly (FIG. 5). The transmit portion 101A ofthe transmitter 100 may indicate a new transmit mode in a preamble orheader of a packet to send to the receiver 102, which notifies thereceiver 102 that the transmitter 100 will use the new transmit mode.The receiver 102 may then switch to the new transmit mode.

The signal may identify the transmit modes in the shortlist explicitly,or may include indices to a table or addresses in a memory in which thetransmit modes are stored. For example, The transmit portion 101B of thereceiver 102 may send transmit mode indices (s*, r*, m*) to thetransmitter 100. The indices indicate entries in a lookup table 108,which is also stored in the transmit or receive portions 101A, 103A ofthe transmitter 100 or elsewhere in the transmitter 100. The transmitportion 101A of the transmitter 100 puts the indices (s*, r*, m*) in apreamble or header of a packet to send to the receiver 102, whichnotifies the receiver 102 that the transmitter 100 will use new s*, r*,and m*. The receiver 102 switches to the new s*, r*, and m*.

The methods above may be a high data rate extension of IEEE 802.11a.These methods may be an efficient mechanism to select a large number ofdata rates. For example, if there are 4 transmit antennas, each with 50Mbits/s, the transmitter may select from a large number of differentdata rates from 6 Mbits/s to 200 Mbits/s.

FIG. 9 is a rate vs. distance graph of a 1×1 SISO system and a 3×3 MIMOsystem. Compared to the SISO system, the MIMO system provides a higherdata rate at a given distance, a farther distance for communication at agiven data rate, and more system and network capacity. Furthermore, moreusers can use the MIMO system.

In alternative embodiments, the receiver 102 may transmit prohibitedtransmit modes, i.e., transmit modes other than those determined to beoptimal. The prohibited modes may include all modes other than thosedetermined to be optimal based on the channel quality statistics, or mayinclude transmit modes determined to be the worst transmit modes basedon the channel quality statistics. The prohibited transmit modes may betransmitted as, e.g., transmit modes, indices to a table, or memoryaddresses. After receiving the prohibited transmit modes, thetransmitter 100 may transmit signals using transmit modes other than theprohibited modes, e.g., by cycling through available modes in the lookuptable 108.

In alternative embodiments, the techniques described above may beimplemented on other types of communication systems, such as, forexample, SISO, SIMO (single-in multiple-out), and MIMO (multiple-inmultiple-out) systems.

A number of embodiments have been described. Nevertheless, it will beunderstood that various modifications may be made without departing fromthe spirit and scope of the application. For example, a 3×3 MIMO systemmay be used. Accordingly, other embodiments are within the scope of thefollowing claims.

What is claimed is:
 1. An apparatus comprising: circuitry configured tocontrol a transmission of a first signal wirelessly to a wirelesscommunication device in accordance with a first transmit mode that isselected from a plurality of transmit modes; circuitry configured toreceive a shortlist from the wireless communication device, theshortlist identifying a subset of the transmit modes, the subset of thetransmit modes including two or more modes that are different from thefirst transmit mode, wherein the wireless communication device isconfigured to generate the shortlist based on a channel quality analysisof a received version of the first signal; and circuitry configured toselect a second transmit mode from the shortlist for a transmission of asecond signal, and selectively cycle through any remaining modes of theshortlist based on a lack of reception of an acknowledgement to thesecond signal, wherein the circuitry configured to control thetransmission is configured to control a transmission of the secondsignal wirelessly to the wireless communication device in accordancewith the second transmit mode or one of the remaining modes.
 2. Theapparatus of claim 1, wherein the plurality of transmit modes comprisesdifferent combinations of one or more of the following: transmissionsignaling schemes, data rate, coding rate, modulation order, andmodulation level.
 3. The apparatus of claim 1, wherein the shortlistidentifies one or more multiple-in-multiple-out (MIMO) transmissionmodes.
 4. The apparatus of claim 1, wherein the channel quality analysiscomprises determining at least one of a signal-to-noise ratio orsignal-to-interference-and-noise ratio associated with the receivedversion of the first signal.
 5. The apparatus of claim 1, wherein theshortlist is included in an acknowledge (ACK) or no acknowledge (NACK)packet.
 6. The apparatus of claim 1, wherein the shortlist compriseslookup table indices corresponding to the two or more modes.
 7. Theapparatus of claim 1, wherein the second transmit mode is selected basedon having a highest data rate within the shortlist.
 8. A methodcomprising: transmitting a first signal wirelessly to a wirelesscommunication device in accordance with a first transmit mode that isselected from a plurality of transmit modes; receiving a shortlist fromthe wireless communication device, the shortlist identifying a subset ofthe transmit modes, the subset of the transmit modes including two ormore modes that are different from the first transmit mode, wherein thewireless communication device is configured to generate the shortlistbased on a channel quality analysis of a received version of the firstsignal; selecting a second transmit mode from the shortlist;transmitting a second signal wirelessly to the wireless communicationdevice in accordance with the second transmit mode; and selectivelycycling through any remaining modes of the shortlist based on a lack ofreception of an acknowledgement to the second signal.
 9. The method ofclaim 8, wherein the plurality of transmit modes comprises differentcombinations of one or more of the following: transmission signalingschemes, data rate, coding rate, modulation order, and modulation level.10. The method of claim 8, wherein the shortlist identifies one or moremultiple-in-multiple-out (MIMO) transmission modes.
 11. The method ofclaim 8, wherein the channel quality analysis comprises determining atleast one of a signal-to-noise ratio or signal-to-interference-and-noiseratio associated with the received version of the first signal.
 12. Themethod of claim 8, wherein receiving the shortlist comprising receivingan acknowledge (ACK) or no acknowledge (NACK) packet.
 13. The method ofclaim 8, wherein the shortlist comprises lookup table indicescorresponding to the two or more modes.
 14. The method of claim 8,wherein the second transmit mode is selected based on having a highestdata rate within the shortlist.
 15. A system comprising: a transceiverconfigured to transmit a first signal wirelessly to a wirelesscommunication device in accordance with a first transmit mode that isselected from a plurality of transmit modes and to receive a shortlistfrom the wireless communication device, the shortlist identifying asubset of the transmit modes, the subset of the transmit modes includingtwo or more modes that are different from the first transmit mode,wherein the wireless communication device is configured to generate theshortlist based on a channel quality analysis of a received version ofthe first signal; and a selector comprising circuitry configured toselect a second transmit mode from the shortlist, wherein thetransceiver is configured to transmit a second signal wirelessly to thewireless communication device in accordance with the second transmitmode, wherein the selector is configured to selectively cycle throughany remaining modes of the shortlist based on a lack of reception of anacknowledgement to the second signal for a transmission of one or morethird signals to the wireless communication device.
 16. The system ofclaim 15, wherein the plurality of transmit modes comprises differentcombinations of one or more of the following: transmission signalingschemes, data rate, coding rate, modulation order, and modulation level.17. The system of claim 15, wherein the shortlist identifies one or moremultiple-in-multiple-out (MIMO) transmission modes.
 18. The system ofclaim 15, wherein the channel quality analysis comprises determining atleast one of a signal-to-noise ratio or signal-to-interference-and-noiseratio associated with the received version of the first signal.
 19. Thesystem of claim 15, wherein the shortlist is included in an acknowledge(ACK) or no acknowledge (NACK) packet.
 20. The system of claim 15,wherein the shortlist comprises lookup table indices corresponding tothe two or more modes.
 21. The system of claim 15, wherein the secondtransmit mode is selected based on having a highest data rate within theshortlist.